Fix Mock 4.0.3 compatibility - Unpatch only once during teardown (#3541)

* #3535 - Unpatch only once during teardown

* EnvVar patching - Fix Python2 bug

* Allow latest version of mock-library

tests/test_s3/test_s3_classdecorator.py

@@ -0,0 +1,18 @@
+import unittest
+
+import boto3
+from moto import mock_s3
+
+
+@mock_s3
+class ClassDecoratorTest(unittest.TestCase):
+    """
+    https://github.com/spulec/moto/issues/3535
+    An update to the mock-package introduced a failure during teardown.
+    This test is in place to catch any similar failures with our mocking approach
+    """
+
+    def test_instantiation_succeeds(self):
+        s3 = boto3.client("s3", region_name="us-east-1")
+
+        assert s3 is not None